The present invention generally relates to an apparatus for processing a microelectronic workpiece. More particularly, the present invention is directed to a processing tool that includes an improved electrochemical processing reactor that may be used to electrochernically etch one or more layers from a microelectronic workpiece. For purposes of the present application, a microelectronic workpiece is defined to include a workpiece formed from a substrate upon which microelectronic circuits or components, data storage elements or layers, and/or micro-mechanical elements are formed. Although the present invention will be described with respect to electrochemical etching, it will be recognized that many of the principles set forth herein are also applicable to other electrochemical tools and reactors.
FIG. 1, labeled “prior art,” illustrates the background art of electrochemical etching. The apparatus shown is a basic electrochemical etching cell. A tank T holds liquid electrolyte E, which is typically an aqueous solution of a salt. Two electrodes, the anode A and the cathode C, are wired to a voltage source such as a battery B. When the apparatus is electrified, metal atoms in the anode A are ionized by the electricity and forced out of the metal into the solution, which, in turn, causes the metal anode A to dissolve into the aqueous solution. The rate of dissolution is proportional to the electric current, according to Faraday's law. Depending on the chemistry of the metals and salt, the metal ions from the cathode either plate the cathode, fall out as precipitate, or stay in solution.
Different types of electrochemical etching apparatus are described in the literature, but most are based on the foregoing principles. In conventional electrochemical etching reactors, the cathode is a shaped tool held close to the anode. The cathode is slowly moved over the face of the workpiece while electrolyte is pumped into the interstitial gap between the cathode and the workpiece, which is connected as the anode. Due to electrical field effects, the highest dissolution rates on the workpiece surface are in those places where the cathode has closely approached the anode surface. The rate falls off as the distance between the anode and the cathode increases.
By choosing proper electrolyte and electrical conditions electropolished surfaces can be achieved in electrochemical etching. As the name implies, electropolishing creates a very smooth mirror-like surface, said to be specular or bright, whose roughness is smaller than a wavelength of light. Unlike a mechanically polished surface, an electropolished surface has no built-up stress left by the high pressures of machining and mechanical polishing. The conductive metal may be selectively or completely etched from the surface of the workpiece. In the microelectronic industry, for example, electrochemical etching is used for through-mask patterning and for removal of continuous thin film conducting metals, such as seed layers, from the surface of a workpiece, such as a semiconductor wafer.
In electrochemical etching processes, the material being removed provides the conductive path for supplying a necessary portion of the processing power. As a result, the removal of material must be performed in a generally controlled manner. Attempts to concurrently remove the entire conductive surface of the workpiece may result in the etching away of portions of the conductive layer located proximate the source of processing power before areas located remote from the processing power source are removed. Remote areas would therefore become electrically isolated from the processing power prior to the completion of the electrochemical etch in those areas. By selectively applying the etching process, the likelihood of the day at a region will be electrically isolated is significantly reduced.
In the foregoing apparatus, material that is removed from surface of the workpiece will migrate to conductive surfaces of the electrode that is used to etch the workpiece material (“the etching electrode”). As the number of workpieces processed increases, the amount of material that collects on the etching electrode will likewise increase. This buildup of conductive material may have a significant effect on the uniformity of the surface of the etching electrode. Additionally, the buildup of material may interfere with the free-flow of electrolyte through nozzle openings of the etching electrode that are provided to supply a flow of electrolyte to the surface of the workpiece.
The non-uniformity resulting from the material build-up alters the gap distance between the anode, formed by the surface of the workpiece, and the cathode formed by the etching electrode. These non-uniformities, in turn, result in a corresponding non-uniformity in the electric field between the workpiece and etching electrode. The electric field variations give rise to uneven etch rates. As the variations in the uniformity of the etch rate increase, so does the chance that portions of the workpiece surface may become electrically isolated from the source of processing power prior to completion of the etching process in those areas. Further, such variations cannot be tolerated in processes that require highly uniform etched surfaces, such as in electrochemical planarization.
Another factor that can affect the uniformity of the current density and, consequently, the uniformity of the etching rate, is the change in the area of the workpiece that is exposed to the etching electrode as the etching electrode is swept across the workpiece. The degree to which this changing area affects the etching rate is dependent on the relative shape of both the workpiece and etching electrode. For example, this etching rate dependency occurs when a circular wafer is swept by a paddle-shaped etching electrode assembly having a rectangular etching electrode. Initially, as the rectangular etching electrode begins to move across the surface of the workpiece, it intersects a first edge of the wafer. In most reactors, the rectangular etching electrode assembly intersects the workpiece at a point that is approximately at the center of the rectangular electrode. As the rectangular etching electrode moves toward the center of the workpiece, the area over which the etching electrode and the workpiece surface are exposed to one another increases. When the rectangular etching electrode is positioned proximate the center of the workpiece, the area of exposure is typically at its maximum value. As the etching electrode continues to move across the workpiece, away from the center of the workpiece, the area of exposure again begins to decrease until the etching electrode completes it movement to the opposite edge of the workpiece. The varying area of exposure between the workpiece and the etching electrode can have a significant detrimental effect on current densities and etch rates and, thus, have a corresponding detrimental effect on the desired results of the etching process.
The present inventors have recognized many of the problems associated with electrochemical etching reactors and processes employing existing microfabrication facilities. One or more of these problems are addressed in the exemplary processing tool set forth herein that includes an improved electrochemical etching reactor.